Log Message:
emulators/openmsx: Add <cstring>, fix *way* leftover file

Several files needed the <cstring> include for functions like memset.
Additional, Tinderbox caught this error:

=== Checking filesystem state after all packages deleted
================================================================
list of extra files and directories in / (not present on clean system
  but present after everything was deinstalled)
34217227226  0  lrwxr-xr-x  1  root  wheel  65  Aug 12 00:13
  usr/local/bin/openmsx ->
   /work/emulators/openmsx/work/.destdir/usr/pkg/openmsx/bin/openmsx

The package was installing a symbolic link from /usr/local/bin to the
destdir!  Luckily this feature is switchable with a variable.  Fixed.